Change the 'libcef_static' target type from static_library to source_set

Using a source_set here keeps the linker on Windows from discarding exported
compilation units that are not directly called from inside libcef_static.

This may also fix linker errors on Linux due to CEF's use of an intermediate
static library (see https://crbug.com/1319006#c2).

BUILD.gn

@@ -406,7 +406,7 @@ test("libcef_static_unittests") {
   ]
 }
 
-static_library("libcef_static") {
+source_set("libcef_static") {
   sources = includes_common +
             gypi_paths.autogen_cpp_includes + [
     "libcef/browser/alloy/alloy_browser_context.cc",

libcef/common/alloy/alloy_main_delegate.cc

@@ -73,15 +73,6 @@ AlloyMainDelegate::AlloyMainDelegate(CefMainRunnerHandler* runner,
                                      CefSettings* settings,
                                      CefRefPtr<CefApp> application)
     : runner_(runner), settings_(settings), application_(application) {
-  // Necessary so that exported functions from base_impl.cc will be included
-  // in the binary.
-  extern void base_impl_stub();
-  base_impl_stub();
-  // Necessary so that exported functions from time_impl.cc will be included
-  // in the binary.
-  extern void time_impl_stub();
-  time_impl_stub();
-
 #if BUILDFLAG(IS_LINUX)
   resource_util::OverrideAssetPath();
 #endif

libcef/common/base_impl.cc

@@ -22,19 +22,6 @@ constexpr const char kCategory[] = "cef.client";
 
 }  // namespace
 
-// The contents of this file are a compilation unit that is not called by other
-// functions in the the library. Consiquently MSVS will exclude it during the
-// linker stage if we don't call a stub function.
-#if defined(COMPILER_MSVC)
-#pragma optimize("", off)
-#endif
-
-void base_impl_stub() {}
-
-#if defined(COMPILER_MSVC)
-#pragma optimize("", on)
-#endif
-
 CEF_EXPORT void cef_trace_event_instant(const char* /* category */,
                                         const char* name,
                                         const char* arg1_name,

libcef/common/time_impl.cc

@@ -9,19 +9,6 @@
 #include <limits>
 #include <tuple>
 
-// The contents of this file are a compilation unit that is not called by other
-// functions in the the library. Consiquently MSVS will exclude it during the
-// linker stage if we don't call a stub function.
-#if defined(COMPILER_MSVC)
-#pragma optimize("", off)
-#endif
-
-void time_impl_stub() {}
-
-#if defined(COMPILER_MSVC)
-#pragma optimize("", on)
-#endif
-
 #if BUILDFLAG(IS_WIN)
 namespace {